1. Select the object you want to be glass
2. Right click and select properties
3. Click Tie to entity
4. Now if you want the glass to be breakable select func_breakable,
if you dont then select func_wall
5. Set the render mode to texture
6. Set the fx amount to a value between 0 and 255, setting it
to 255 will make it non-transparent, 90 is a good setting for
glass, the more transparent glass is the more the fps will drop
in game.
7. Go and smash your glass if you dare